1. Field of the Invention
This invention relates in general to check valve assemblies, especially those for preventing upward fluid flow in drill string used in earth boring.
2. Description of the Prior Art
Oil and gas wells are conventionally drilled by rotating a string of drill pipe that supports a bit. Drilling fluid, known as "mud", is pumped down the inner passage of the drill string, through the bit and up the annular passage between the borehole wall and drill pipe. The mud circulates cuttings to the surface and cools the bit. Also, the weight of the mud is adjusted to provide a bottom hole hydrostatic pressure greater than the formation pressure to prevent a "blowout".
If this condition occurs when the surface pump is disconnected from the drill pipe (as when a joint of pipe is being added or the pipe being removed or replaced during a "trip"), a blowout is more likely because the pump cannot be used to exert added pressure to the fluid inside the pipe. Also, heavier fluids with "weighting" materials cannot be quickly pumped through the system to help avert a blowout.
"outside" blowout preventers, which have been marketed and used for years, are presently used to grip the drill pipe at the surface and seal the annulus between the pipe and the wall of the hole. These devices do not prevent the upward flow of mud inside the pipe during a blowout.
"Inside" blowout preventers to prevent flow up the inner passage of the drill string are shown in U.S. Pat. Nos. 3,850,191; 3,850,194; 4,040,441; 4,049,015; 4,088,298; and 4,108,203. In certain of these patents, the device comprises a sub for connection into the string. The sub has a longitudinal passage, with a recess or side pocket on one side and a seat in the passage above the side pocket. A ball is carried in the side pocket during normal drilling. It moves into the longitudinal passage and seats if substantial upward flow exists during a blowout. An equalizing passage, connecting the base of the side pocket with the longitudinal passage below the side pocket, determines the rate of upward flow which will move the ball into the longitudinal passage. The cross-sectional area of the equalizing passage prevents seating of the ball from the normal upward flow caused by running the drill string into the well.
Erosion is a continuing problem with inside blowout preventers of the above prior art description. High flow rates and solids or sand contained in the mud cause erosion of the seat and passages inside the blowout preventer, preventing long-term effective operation. Experience indicates that the ball tends to move partially into the longitudinal passage even during downward fluid flow. In this position the ball causes a flow restriction of varying area due to oscillation of the ball. Turbulent flow and erosion of the passage and ball inevitably result.
This invention relates to solutions to the above problems of the prior art devices.
This invention, like the prior art inside blowout preventers previously discussed, utilizes a ball and seat in the primary or longitudinal passage to prevent the upward flow of fluid. A side pocket or recess receives the ball during downward flow to remove the ball from the primary flow path.
Unlike the prior art assemblies, the recess is appreciably larger in diameter than the ball, enabling fluid to flow downward past the ball, through an equalizing passage and back into the longitudinal passage downstream from the recess and ball. Consequently, when fluid flows downward in a drill string during drilling, some of the fluid flows past the ball in the recess to prevent movement of the ball, even partially, into the primary or longitudinal passage.
When fluid flows upwardly in the drill string, upward flow through the equalizing passage into the recess urges the ball into the longitudinal or primary passage and against a seat.
The check valve assembly is encompassed within an inner member or replaceable core contained by a tubular housing. Inside the core is an offset throat that forms a portion of the longitudinal or primary passage that contains the seat for the ball. The side pocket or recess communicates with the throat and an annular equalizing passage between the replaceable core and the tubular housing.
Addition features, objects and advantages of the invention will become apparent in the following description.